The importance of endcapping or terminating polycarbonates with certain end or terminal groups is well known. Polycarbonates which are not so endcapped are generally insufficiently heat stable since the free phenolic end groups provide reactive sites which are generally detrimental to the thermal stability of the polycarbonate. Well known and conventionally used endcapping agents include phenol compounds such as paratertiary butyl phenol, Chroman-I, paracumylphenol, and phenol itself. The prior art also discloses other types of compounds that are effective endcapping agents for the carbonate polymers. These endcapping agents include the alkyl phenols disclosed in U.S. Pat. No. 4,269,964 and Japanese patent publication No. 34992/76; the alkenyl amines disclosed in U.S. Pat. No. 3,085,992; the amides disclosed in U.S. Pat. No. 3,399,172; aniline and methylaniline as disclosed in U.S. Pat. No. 3,275,601; and the primary and secondary amines disclosed in U.S. Pat. No. 4,001,184. Other endcapping agents described in the prior art include the aromatic amines disclosed in U.S. Pat. No. 3,028,365; and the ammonium compounds, ammonia, primary cycloalkyl amines, and primary aliphatic or alkyl amines disclosed in U.S. Pat. No. 4,111,910.
Various functionalized phenyl maleimides have been used to control polyamide molecular weight. These resins are claimed to be useful in ultraviolet cross-linkable compositions, see U.S. Pat. No. 4,645,822, assigned to Bayer. In existing maleimide capped resins, the highly reactive maleimide group is left exposed after isolation of resin. It is likely that many of the unsaturated end groups will react even before the polymer melts. In addition, the maleimide grouping is relatively toxic and therefore handling of the material is somewhat restrictive.
A novel material has been discovered which bypasses the above problems. A masked maleimide endcapped polycarbonate is prepared from the substituted phenolic chain capping material. This material is extremely stable and not abnormally toxic at room temperature. However, when heated to relatively high temperaures as in an extruder, the masking portion of the molecule is cleaved, therefore leaving the maleimide endcapped polycarbonate available for further reaction with itself or with other polymer systems thereby providing a new route to novel copolymers through a melt grafting process.